Most offshore oil and gas production is conducted from structures (commonly termed "platforms") supported on the seafloor and extending upward to a drilling and production deck situated above the ocean surface. A key constraint in the design of such platforms is that dynamic amplification of the platform's response to waves must be avoided. Failure to substantially avoid dynamic amplification will result in a reduction of the fatigue life of the platform and in extreme cases can result in failure of key structural platform components. Dynamic amplification of the platform's response is avoided by designing the platform such that each of its natural vibrational periods fall outside that range of wave periods representing waves of significant energy. For most offshore locations the range of natural vibrational periods to be avoided is from 7 to 25 seconds, this representing the range of wave periods occurring with the greatest frequency. The several modes of platform vibration which are generally of greatest concern in platform design are pivoting (commonly termed "sway") of the structure about a joint, flexure ("bending") of the structure along its height, and torsion about the structure's vertical axis.
For water depths up to about 300 meters, the technology for avoiding dynamic amplification of an offshore structure's wave response is well developed. Nearly all existing offshore structures used in such water depths are rigidly secured to the ocean bottom and stiffened to cause the various natural vibrational periods of the structure to be less than about 7 seconds. Such offshore structures are referred to as "rigid structures." The most common rigid structure used in offshore oil production is a tubular steel space-frame secured to the ocean floor by a pile-type foundation. An alternate rigid structure, employed most extensively in the North Sea, is the concrete gravity structure. Concrete gravity structures include a caisson which rests on the ocean floor. One or more towers are rigidly secured to the caisson and extend upward to a drilling and production deck above the ocean surface. Foundation skirts extend downward from the caisson to transmit lateral environmental loads into the ocean floor. The caisson and skirts act under the submerged weight of the structure to establish a gravity foundation rigidly supporting the tower on the ocean floor.
As water depths exceed 300 meters, the volume of structural material required to maintain sufficient platform stiffness to retain the natural vibrational periods of a rigid structure below 7 seconds increases rapidly with depth. As a result, the cost of rigid structures becomes increasingly depth sensitive in water depths beyond 300 meters. It has been suggested that for even the richest offshore oil fields the use of a rigid structure could not be economically justified in water depths exceeding about 420 meters due to the constraint imposed by the maximum permissible natural vibrational period.
For deep water applications, it has been proposed to depart from conventional rigid structure design and develop platforms having a fundamental period greater than the range of periods of ocean waves containing significant energy. Such platforms, termed "compliant structures," do not rigidly resist waves and other environmental forces, but instead compliantly resist environmental loads primarily by their own inertia, undergoing significant lateral motion at the ocean surface. The mode shape associated with the fundamental period of a compliant structure is typically achieved either by pivoting of the structure about a joint or by bending over some length in the structural system itself. It is normally impractical to render the periods of second and higher modes compliant, requiring that these periods be kept below about 7 seconds to prevent dynamic amplification of the higher modes. Thus, compliant structures are characterized by the fact that the range of ocean-wave periods containing significant energy is straddled by the fundamental period on the high side (above about 30 seconds), and by all remaining periods on the low side (below about 7 seconds). The use of a compliant offshore structure effectively removes the upper bound on the fundamental period, thus avoiding the most troublesome design contraint of rigid structures. This greatly reduces the increase in the volume of structural material, and hence cost, required for a given increase in water depth.
Compliant structures must be provided with some mechanism for countering lateral displacement resulting from the action of wind, waves and currents. Countering such lateral displacement is termed "stabilization." Stabilization is accomplished in existing compliant offshore structures in a variety of manners. In one class of compliant offshore structures, including tension leg platforms and buoyant towers, stabilization is provided by buoyancy. Such structures include a buoyant portion typically located either at the ocean surface or just below the wave zone. As environmental forces displace the platform from a vertical orientation, the buoyant force acting on the buoyant portion establishes a righting moment which acts to restore the structure to a vertical orientation. A significant disadvantage of buoyant structures is that the large buoyancy chambers they require greatly increase the expense of the structure. Additionally, these buoyancy chambers must be located at or near the ocean surface, increasing the cross-sectional area of the structure exposed to environmental forces. This results in increased loading, requiring a stronger structure than would otherwise be necessary. A typical tension leg platform is shown in U.S. Pat. No. 4,428,702, issued Jan. 31, 1984. A typical buoyant tower is shown in U.K. Pat. No. 2,066,336B, issued Nov. 2, 1983.
In a second type of compliant structure, the guyed tower, the platform deck is supported on a slender space-frame structure extending from the ocean bottom to the ocean surface. A plurality of catenary guylines extend radially outward from an upper portion of the space-frame structure to the ocean bottom. The guylines provide the necessary stabilization. A major disadvantage of guyed towers is that the guyline system is expensive to fabricate, deploy and maintain. In certain applications the guylines also present an obstacle to navigation and fishing in the vicinity of the platform. A typical guyed tower is detailed in U.S. Pat. No. Re. 32,119, issued Apr. 22, 1986.
A third type of compliant structure, known as the compliant piled tower, uses flex plies to provide stability. The compliant piled tower is a rigid tower structure having piles extending upward along its periphery to a preselected elevation where the piles are grouted or otherwise rigidly secured to the tower. The tower is supported laterally at its lower end by the flex piles, but is permitted to slide vertically along the flex piles and rotate about its lower end. In response to movement of the tower away from the vertical, the piles establish a righting moment (couple) acting at the point of pile attachment. This provides the stabilization necessary to restore the tower to a vertical orientation. A disadvantage of the compliant piled tower is that design and installation are complicated by conflicting demands placed on the flex piles, which must be flexible enough to achieve compliant behavior, yet stiff enough to withstand pile-driving stresses during installation. Another disadvantage of the compliant piled tower is that a portion of each flex pile must be driven after the tower is in place. This is expensive and extends the duration of the required installation window when the structure is vulnerable to damage by storms. One form of compliant piled tower is detailed in U.S. patent application Ser. No. 806,055, filed Dec. 5, 1985.
It would be desirable to develop a compliant offshore platform which does not rely on positive buoyancy, guylines or fixed piles to counter the lateral forces imposed by wind, waves, and ocean currents.